The article, "Enhanced Differential GPS Carrier-Smoothed Code Processing Using Dual Frequency Measurements," Proceedings of ION GPS-98, The 11.sup.th Int. Tech. Meeting of the Satellite Div. of the Inst. of Navigation, Nashville, Tenn., Sept. 15-18, 1998, by P. Y. Hwang, G. A. McGraw and J. R. Bader, is herein incorporated by reference.
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency f.sub.L1 of 1575.42 MHz, and an L2 signal having a frequency f.sub.L2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code) is reserved for military uses.
GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. Additional satellite measurements can be used to improve the position solution.
Differential GPS (DGPS) utilizes a base station located at a known position and one or more remote GPS receivers. The base station receives GPS positioning signals from the satellites and calculates predicted measurements based upon the known base station location. Based upon differences between the predicted base station measurements and the actual measurements, the base station transmits corrections to the remote GPS receiver. The remote GPS receiver uses the corrections and received GPS satellite signals to calculate its position more accurately.
Differential GPS architectures may be divided into two broad classes, local area and wide area. While variations in the ionospheric refraction (iono for short) are a major concern over the larger spatial domains covered by wide-area architectures, they are generally ignored in local-area systems since the iono component is strongly correlated over the smaller domains they cover. However, time variations in the iono do affect the smoothing of measurement noise, which is an aspect not always dealt with satisfactorily in local-area systems.
The technique of combining GPS code and carrier pseudorange information in a complementary filter to attenuate code multipath and tracking noise is known and is the basis for high-accuracy local-area code differential GPS systems, such as the FAA's Local Area Augmentation System (LAAS). A well-known limitation to this carrier smoothing processing technique is divergence in the iono refraction between code and carrier, which gives rise to a residual ranging error that is proportional to the smoothing filter time constant. Thus, in such code differential GPS systems, there is a trade-off between the iono-induced smoothing error and the amount of attenuation that can be achieved of slowly-varying code errors such as multipath. The current LAAS concept compensates for the iono divergence smoothing error by requiring that the reference and airborne receivers use the same steady state smoothing time constant, but this has adverse implications for continuity of function and availability.